Stars of average mass, such as our Sun, undergo fusion. The most massive element that can result to the process is Helium.

***In our Sun and similar stars, Hydrogen is being fused into Helium. At some stage, Helium is fused into Carbon, then Oxygen, etc.The fusion process requires positively charged nuclei to move very close to each other before they can unite. But withincreasing atomic mass and hence, increasing positive charge of the nuclei, the electric repulsion between the nuclei becomesstronger and stronger.